Abstract

To evaluate the possible physiology of dinosaurs, comparisons must be made with their closest living relatives: birds and
crocodilians. Although crocodilians maintain ectothermic metabolic rates and have anatomy reflective of this, modern birds
achieve high, endothermic metabolic rates through specialised soft tissues supported by unique skeletal attributes. Finding
similar shared characters in dinosaurs that are functionally linked to metabolic rates in birds or crocodilians allows plausible
reconstruction of dinosaur physiology. Examinations of dinosaur remains reveal no structures with clear functional association
with bird‐like respiratory or metabolic physiology, and in some cases indicate crocodilian‐like anatomy. Consequently, dinosaurs
were most likely ectothermic, with resting and maximal metabolic rates that were lower than those of modern mammals or birds.
However, given the favourable Mesozoic climatic conditions, most dinosaurs were probably able to maintain high, constant body
temperatures through behavioural or inertial thermoregulation.

Key Concepts:

Reconstructing the biology of extinct forms relies on comparison with living taxa that share the same specialised features
linked to specific function.

Stable body temperature can be achieved through behavioural mechanisms or through virtue of large mass, and need not rely
on a particular metabolic strategy.

The closest living relatives of dinosaurs are birds and crocodilians, which have widely different metabolic rates supported
by different respiratory and skeletal anatomy.

Some dinosaur remains preserve evidence, such as postcranial pneumaticity, that may be superficially suggestive of modern
bird‐like respiratory anatomy, but they lack other features critical for the ability to ventilate bird‐like lungs or achieve
bird‐like aerobic capacity.

No dinosaur remains show evidence of respiratory turbinates, a skeletal character functionally associated with modern endothermy.

Endothermy was not likely achieved in dinosaurs, but was first present in mid‐Cretaceous birds.

Some dinosaurs may have increased aerobic capacity using a crocodilian‐like ventilatory mechanism.

Nasal passages of endotherms. Respiratory turbinates (rt) are complex bony or cartilaginous structures in the nasal cavities
of mammals (top left) and birds (bottom left), which help limit heat and water loss during breathing. These structures are
missing in all modern reptiles. Also shown are cross‐sections of representative birds (bottom right) and mammals (top right).

Figure 2.

Relationship between cross‐sectional area of the nasal passage and body mass in modern endotherms and ectotherms. The larger
dimensions in endotherms compensate for the increase in resistance to air flow presented by the respiratory turbinates. Also
plotted are three genera of Late Cretaceous dinosaurs, the hadrosaurid Hypacrosaurus and the theropods Nanotyrannus and Ornithomimis.
Data from Ruben et al..

Figure 3.

Lung ventilation in modern birds. Specialised hinge joints between vertebral and sternal ribs of modern birds coupled with
the bifurcate end of sternal ribs (bottom right, * on SR, example from ostrich) on the sternum allow an up‐and‐down rocking
motion that is responsible for the ventilation of the airsacs (top). The sternum of modern birds is therefore characterised
by a robust lateral edge with transversely oriented joint facets that articulate with the sternal ribs (middle) that articulate
with shallow depressions on the sternal ribs (bottom left, arrow head). Bar, 1 cm.

Figure 4.

Lung ventilation in dinosaurs. Dinosaurs lacked the specialised hinged ribs and sternocostal joints associated with lung ventilation
in modern birds. However, preserved soft tissues in the abdominal cavity of the compsognathid Sinosauropteryx (left) and the
maniraptoran Scipionyx (right; bottom image under ultraviolet illumination) suggest that theropods may have piston‐like liver
movements to enhance lung ventilation. Bar, 1 cm.

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